fix(attachments): flush scope on updating attachments

[JoshuaMoelans]

Fixes #1933

Updates to attachments weren't flushed to the scope (only when adding it with sentry_attach_file). This caused updates to attachments to only apply on non-fatal events, since the native backend only knows about the state of the attachment from right before attaching it.

This fixes both questions in the original issue; attachment names are synced & so are the content_types, which initially made Sentry backend interpret the file as the default application/octet-stream which gets a Preview button (whereas the application/ztsd would not)

From the flow that UE hits on adding attachments, we see the same attach -> set_filename -> set_content_type flow that we now fix with scope syncing:

void FGenericPlatformSentrySubsystem::AddFileAttachment(TSharedPtr<ISentryAttachment> attachment)
{
	TSharedPtr<FGenericPlatformSentryAttachment> platformAttachment = StaticCastSharedPtr<FGenericPlatformSentryAttachment>(attachment);

	sentry_attachment_t* nativeAttachment =
		sentry_attach_file(TCHAR_TO_UTF8(*platformAttachment->GetPath()));

	if (!platformAttachment->GetFilename().IsEmpty())
		sentry_attachment_set_filename(nativeAttachment, TCHAR_TO_UTF8(*platformAttachment->GetFilename()));

	if (!platformAttachment->GetContentType().IsEmpty())
		sentry_attachment_set_content_type(nativeAttachment, TCHAR_TO_UTF8(*platformAttachment->GetContentType()));

	platformAttachment->SetNativeObject(nativeAttachment);

	attachments.Add(platformAttachment);
}

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.79%. Comparing base (5bac907) to head (ba60934).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1934      +/-   ##
==========================================
+ Coverage   75.74%   75.79%   +0.05%     
==========================================
  Files          93       93              
  Lines       22128    22155      +27     
  Branches     3939     3945       +6     
==========================================
+ Hits        16760    16792      +32     
+ Misses       4484     4477       -7     
- Partials      884      886       +2     

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[JoshuaMoelans]

@sentry review